(Supported in part by NIH GMS 30758 to G. Sluder and NIH GMS 40198 to C.L. Rieder). To better understand how the cell coordinates centrosome reproduction with nuclear events during the cell cycle we tested the ability of centrosomes to repeatedly reproduce as a function of cell cycle stage and level of Cdk-1B activity in sea urchin zygotes. Throughout the course of this study the HVEM was used as a tool to assay for centriole replication and numbers in experimentally treated cells. We have previously shown that such same-cell correlative LM and EM studies on sea urchin zygotes can only realistically be conducted by HVEM or IVEM of thick (0.50 (m) serial sections. The data from this study reveals that centrosomes under repeated rounds of replication when the S phase of the cell cycle is prolonged, and they point to a necessary "licensing" event that occurs during this phase that allows centrioles to reproduce again. Hinchcliffe, E.H., G.O. Cassels, C.L. Rieder and G. Sluder. (1997) Control of centrosome reproduction in sea urchin zygotes: Cell cycle stage and not CDK1 activity coordinates centrosome duplication with nuclear events of the cell cycle. J. Cell Biol., (in press). Hinchcliffe, E.H., C.L. Rieder and G. Sluder. (1997) A "liscensing" event during S-phase is required for centrosome reproduction in sea urchin zygotes. Mole.Biol. Cell 8: